A method of forming images by diffusion transfer using silver salts such as silver halide is well known. In accordance with this method, an imagewise exposed light-sensitive silver halide emulsion is first treated with an aqueous alkali solution containing a developing agent, a silver halide solvent and a film-forming agent (tackifier) to reduce the exposed silver halide grains to silver with the developing agent while unexposed silver halide grains are converted into a transferable silver complex salt by the silver halide solvent. The silver complex salt is then transferred by diffusion to a silver precipitating agent-containing layer (image-receiving layer) superimposed on the above emulsion layer by imbibition, and the silver complex salt is reduced by the developing agent and the silver precipitating agent to obtain a silver image.
In the practice of the above method, there is used, for example, a film unit comprising a light-sensitive element comprising a support and a light-sensitive silver halide emulsion layer provided thereon; an image-receiving element comprising a support and an image-receiving layer containing a silver-precipitating agent provided on the support, and a processing element comprising ruptuable containers accomodating an active aqueous alkali solution containing a silver halide solvent, a developing agent and a film-forming agent. In the first step, the emulsion layer of the light-sensitive element is exposed imagewise, and then the light-sensitive element and the image-receiving element are superimposed in such a manner that the emulsion layer and the image-receiving layer of the image-receiving element face each other, and are then passed between a pair of rolls so that the processing element is broken and the viscous aqueous alkali solution is spread between the two elements. The superimposed and alkali treated elements are allowed to stand for a predetermined time and the image-receiving element is peeled apart from the light-sensitive element whereupon the desired image is formed on the image-receiving layer.
If, however, the image formed by the diffusion transfer method is stored for a long time, discoloration or fading of the image will occur. This discoloration or fading is undesirable for a recording material.